Microcavity plasma devices with non-uniform cross-section microcavities

ABSTRACT

An embodiment of the invention IS an array of microcavity plasma devices The array includes a first metal film electrode with a plurality of non-uniform cross-section microcavities therein that are encapsulated in oxide A second electrode is a thin metal foil encapsulated in oxide that is bonded to the first electrode A packaging layer contains gas or vapor in the non-uniform cross-section microcavities To make such device, photoresist is patterned to encapsulate the anodized foil or film except on a top surface at desired positions of microcavities A second anodization or electrochemical etching is conducted to form the non-uniform cross-section sidewall microcavities cavities After removing photoresist and metal oxide, a final anodization lines the walls of the microcavities with metal oxide and fully encapsulates the metal electrodes with metal oxide.

PRIORITY CLAIM AND REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 from priorprovisional application Ser. No. 61/000,389, which was filed on Oct. 25,2007.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under contract numberFA9550-07-1-0003 awarded by Air Force Office of Scientific Research. Thegovernment has certain rights in the invention.

FIELD

A field of the invention is microcavity plasma devices (also known asmicrodischarge devices) and arrays of microcavity plasma devices.

BACKGROUND

Microcavity plasma devices produce a nonequilibrium, low temperatureplasma within, and essentially confined to, a cavity having acharacteristic dimension d below approximately 500 μm. This new class ofplasma devices exhibits several properties that differ substantiallyfrom those of conventional, macroscopic plasma sources. Because of theirsmall physical dimensions, microcavity plasmas normally operate at gas(or vapor) pressures considerably higher than those accessible tomacroscopic devices. For example, microplasma devices with a cylindricalmicrocavity having a diameter of 200-300 μm (or less) are capable ofoperation at rare gas (as well as N₂ and other gases tested to date)pressures up to and beyond one atmosphere.

Work done by University of Illinois researchers is disclosed in U.S.Published Application Number 20070170866, to Eden, et al., which isentitled Arrays of Microcavity Plasma Devices with DielectricEncapsulated Electrodes. That application discloses microcavity plasmadevices and arrays with thin foil metal electrodes protected by metaloxide dielectric. The devices and arrays disclosed are based upon thinfoils of metal that are available or can be produced in arbitrarylengths, such as on rolls. A method of manufacturing disclosed in theapplication discloses a first electrode pre-formed with microcavitieshaving the desired cross-sectional geometry. Pre-formed screen-likemetal foil, e.g. Al screens used in the battery industry, can be usedwith the disclosed methods. Oxide is subsequently grown on the foil,including on the inside walls of the microcavities (where plasma is tobe produced), by wet electrochemical processing (anodization) of thefoil. As disclosed in the application, providing a metal thin foil withmicrocavities includes either fabricating the cavities in metal foil byany of a variety of processes (laser ablation, chemical etching, etc.)or obtaining a metal thin foil with pre-fabricated microcavities from asupplier. A wide variety of microcavity shapes and cross-sectionalgeometries can be formed in metal foils according to the methoddisclosed in the application.

More recent work by University of Illinois researchers discloses buriedcircumferential electrode microcavity plasma device arrays and aself-patterned wet chemical etching formation method includingcontrolled interconnections between. These results are disclosed in Edenet al., U.S. patent application Ser. No. 11/880,698, filed Jul. 24,2007, entitled Buried Circumferential Electrode Microcavity PlasmaDevice Arrays, and Self-Patterned Formation Method, which has beenpublished as WO 08/013,820 on Jan. 31, 2008 and as US 2008-0185579 onAug. 7, 2008. In a disclosed method of formation in that application, ametal foil or film is obtained or formed with microcavities (such asthrough holes), and the foil or film is anodized to form metal oxide.One or more self-patterned metal electrodes are automatically formed andburied in the metal oxide created by the anodization process. Theelectrodes form in a closed circumference (a ring if the cavity shape iscircular) around each microcavity, and can be electrically isolated orconnected. Prior to processing, microcavities (such as through holes) ofthe desired shape are produced in a metal electrode (e.g., a foil orfilm). The electrode is subsequently anodized so as to convert virtuallyall of the electrode into a dielectric (normally an oxide). Theanodization process and microcavity placement determines whetheradjacent microcavities in an array are electrically connected or not.

Microcavity plasma devices fabricated in the metal/metal oxidestructures described above are inexpensive, flexible and durable.Self-assembly processes can be used to automatically form the buriedelectrodes via anodization, as described above. However, priormicrocavity plasma devices formed by semiconductor fabricationtechniques in semiconductors and other materials have offered morecontrol over the cross-sectional geometry (shape) of the microcavitiesthan the anodization processes provided prior to the present invention.A tapered microcavity is provided in Eden, et al. U.S. Pat. No.7,112,918, Sep. 26, 2006, which is entitled Microdischarge Devices andArrays Having Tapered Microcavities. The tapered microcavity providesoperational advantages, including improved extraction of light producedby plasma generated within the microcavity. However, the angle of thetapered sidewall of microcavities in silicon, for example, is fixed bythe crystalline structure of the semiconductor.

SUMMARY OF THE INVENTION

An embodiment of the invention is an array of microcavity plasma deviceshaving microcavities controllable, non-uniform cross-sections. The arrayincludes a first electrode that is a thin metal foil or film having aplurality of non-uniform cross-section sidewall microcavities therein,each of which is encapsulated in oxide. A second electrode is a thinmetal foil, encapsulated in oxide, that is bonded to the firstelectrode, the oxide preventing contact between the first and secondelectrodes. A packaging layer seals discharge medium (a gas or vapor)into the microcavities.

A method for forming an array of microcavity plasma devices begins withpre-anodizing a metal foil or thin film. Photoresist is patterned ontothe anodized metal foil or film to encapsulate the anodized foil or filmexcept on a top surface at the desired positions of microcavities. Asecond anodization is then conducted to form the microcavities withsidewall profiles that can be controlled precisely.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-1F illustrate a preferred embodiment method for forming an arrayof microcavity devices with microcavity sidewalls having a controllableprofile;

FIG. 2 illustrates a continuous range for microcavity cross-sectionalprofiles that are available with methods of the invention;

FIG. 3A is a schematic diagram of another array of microcavity plasmadevices of the invention;

FIG. 3B is a schematic diagram of an addressable array of microcavityplasma devices of the invention;

FIG. 4 presents voltage-current (V-I) characteristics of an array ofmicrocavity plasma devices of the invention operating in 400, 500, 600and 700 Torr of Ne;

FIG. 5 presents V-I characteristics of an array of microcavity plasmadevices of the invention operating in Ne/Xe mixtures at a total pressureof 400 Torr and Xe concentrations of 10, 20, 30, 40, and 50%; and

FIGS. 6A and 6B show a schematic cross-section of another array ofmicrocavity plasma devices of the invention, illustrating the formationof microcavities with curved sidewalls.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides an improved variation of the methods anddevices disclosed in U.S. patent application Ser. No. 11/880,698(incorporated by reference herein) that allows the formation ofmicrocavity plasma devices and arrays having microcavities withcontrollable sidewall profiles. The non vertical sidewall microcavitiesin arrays of the invention can have various predetermined shapes, andare formed by a variation of the wet chemical process disclosed in the'698 application. The entire process of forming the microcavities and“wiring them”—producing electrodes and interconnections—can be realizedin an inexpensive, wet chemical process. In the present invention, thecross-sectional geometry of the microcavities can be continuously variedfrom a “bowl” (concave) shape to a pure linear taper. Fabricationmethods of the invention can be controlled to produce a predetermineddesired shape in the sidewall of the microcavity. This ability toproduce a predetermined shape has been previously provided to a limiteddegree in microcavity plasma devices fabricated by semiconductorfabrication techniques, but not in the inexpensive arrays of microcavityplasma device arrays fabricated in metal/metal oxide structures. See,Eden, et al. U.S. Pat. No. 7,112,918, Sep. 26, 2006, which is entitledMicrodischarge Devices and Arrays Having Tapered Microcavities.

The present invention extends the advantages offered by the taperedmicrocavities in the '918 patent to the metal/metal oxide device arraysthat are formed by inexpensive wet chemical formation processes.Microcavity plasma device arrays of the invention provide advantages fortailoring and optimizing emission and the operating characteristics ofthe array of microcavities. The ability to produce microcavities havinga predetermined sidewall shape allows for tailoring and optimizing theefficiency and operating parameters (excitation voltage, frequency, gaspressure, etc.) of an array of microplasma devices. Another benefit ofcontrolling the cross-sectional profile of the microcavity is theability to optimize extraction of photons (produced by the microplasma)from the microcavity.

In addition, tapered sidewall microcavities provide a large positivedifferential resistance that decreases power consumption while improvingthe linearity of the V-I characteristics. This characteristic permitsself-ballasting of the devices and simplifies external controlcircuitry. The thin sheet metal/metal oxide arrays reported prior to theinvention offer many advantages, including ease of fabrication,transparency, and flexibility. These advantages are retained by arraysof the invention, which also provide the advantages offered bynon-uniform cross-section microcavities. Microdischarge devices withtapered cavities also exhibit an increase in surface area relative to aconventional planar structure, thereby enabling modification of theelectrical properties of devices. In addition, increased output(radiant) efficiencies are obtained by coating the tapered side wallswith an optically reflective conductive coating or a coating with arelatively small work function. Arrays of non-uniform cross-sectionmicrocavity plasma devices produce higher output power and exhibitignition characteristics superior to those of otherwise similar arrayswith uniform cross section microcavities having vertical sidewalls. Theprimary reason for this improved performance is the ability to shape thecavity sidewalls so as to optimize the electrical field profile withinthe microcavity.

An example embodiment array of microcavity devices of the inventionincludes a first electrode, the first electrode being a thin metal foilhaving a plurality of non-uniform cross-section microcavities thereinthat are encapsulated in oxide. A second electrode is a thin metal foilencapsulated in oxide that is bonded to the first electrode, and theoxide prevents contact between the first and second electrodes. Apackaging layer seals the discharge medium (a gas or vapor or mixturethereof) into the microcavities. Exemplary microcavities includemicrocavities having bowl style sidewalls or sidewalls with lineartapers. The microcavities in preferred embodiment arrays of microcavitydevices have a predetermined desired sidewall shape.

A preferred embodiment fabrication process of the invention includespre-anodization of a metal foil or thin film. The parameters of thepre-anodization determine the thickness of the metal oxide formed inpre-anodization which is the primary factor determining the shape of theresulting microcavity. After pre-anodization, photoresist (PR) ispatterned onto the anodized metal foil or film to encapsulate thepartially anodized foil or film except on the top surface at the desiredpositions of microcavities. Encapsulating the foil or film withphotoresist, including the back side (and edges), ensures that a secondanodization of the foil will not occur uniformly with respect to thefront and rear surfaces of the foil. A second anodization is thenconducted to form microcavities having a desired sidewall shape. Themicrocavities form with non uniform cross-section because anodizationfrom the rear surface of the foil has been blocked by the PR coating.The exact shape of the cavity produced is a function of the foilthickness, initial anodization time (and, hence, oxide thickness), andthe second anodization time.

Devices of the invention are amenable to mass production techniqueswhich may include, for example, roll to roll processing to bond togetherthe first and second thin layers with buried electrodes. Embodiments ofthe invention provide for large arrays of microcavity plasma devicesthat can be made inexpensively because they are literally fabricatedfrom aluminum foil by wet chemical processing. Also, exemplary devicesof the invention are formed from thin layers that are flexible and areat least partially transparent in the visible region of the spectrum.

The structure of preferred embodiment microcavity plasma devices of theinvention is based upon foils (or films) of metal that are available orcan be produced in arbitrary lengths, such as on rolls. In a method ofthe invention, a pattern of microcavities is produced in a metal foilthat is subsequently anodized, thereby resulting in microcavities in ametal-oxide (rather than the metal) with each microcavity surrounded (ina plane transverse to the microcavity axis) by a buried metal electrode.During device operation, the metal oxide protects the microcavity andelectrically isolates the electrode from the plasma within themicrocavity.

A second metal foil is also encapsulated with oxide and can be bonded tothe first encapsulated foil. The second metal foil forms a secondelectrode(s). For one preferred embodiment microcavity plasma devicearray of the invention, no particular alignment is necessary duringbonding of the two encapsulated foils. In another embodiment of theinvention, the second electrode comprises an array of thin parallelmetal lines buried in the metal-oxide. The entire array, comprising twometal-oxide sheets with buried electrodes, can be sealed with thinglass, quartz, or even plastic windows, for example, with the desiredgas or gas mixture sealed within.

Preferred materials for the metal electrodes and metal oxide arealuminum and aluminum oxide (Al/Al₂O₃). Another exemplary metal/metaloxide material system is titanium and titanium dioxide (Ti/TiO₂). Othermetal/metal oxide materials systems will be apparent to artisans.Preferred material systems permit the formation of microcavity plasmadevice arrays of the invention by inexpensive, mass productiontechniques such as roll to roll processing.

Preferred embodiments will now be discussed with respect to thedrawings. The drawings include schematic figures that are not to scale,which will be fully understood by skilled artisans with reference to theaccompanying description. Features may be exaggerated for purposes ofillustration. From the preferred embodiments, artisans will recognizeadditional features and broader aspects of the invention. The preferredembodiment devices and methods of fabrication discussed concern Al/Al₂O₃arrays of microcavity plasma devices, but other metal and metal oxidescan also be used, such as titanium and titanium dioxide.

FIGS. 1A-1F illustrate a preferred embodiment method for forming anarray of microcavity devices with non-uniform cross-sectional geometriesof the invention. The method is capable of producing microcavitieshaving a desired sidewall shape, which can range from a bowl-style shapeto a linear taper. The present process has been used in experiments toform example devices, and artisans will appreciate broader aspects ofthe invention from the example experiments. The basic method of FIGS.1A-1F will be discussed along with experimental details. The particulardimensions, conditions and durations of the experiments do not limit theinvention, but provide a specific example embodiment method that willproduce an array of microcavity plasma devices in which themicrocavities have a predetermined (desired) sidewall shape.

In FIG. 1A, a metal foil 6 is provided and the foil 6 is pre-anodized inFIG. 1B to form a coating of metal oxide 8. It is important to note thatalthough the metal oxide is referred to as “a coating” on the foil, inreality a portion of the foil has been converted chemically into anoxide. A typical experimental process used an Al foil of about 30 μmthickness, although foils with thicknesses above 120 μm have also beenprocessed successfully. The pre-anodization of FIG. 1B is important indetermining the shape of the resultant microcavities that are formedlater. With metal foils of about 30 μm, experiments successfully used apre-anodization time of as little as about 1 min. and up to about 1hour. Typically, the pre-anodization process occurred in 0.3 M oxalicacid at a temperature of 15° C. and a voltage of 40 V. The thickness ofthe metal oxide (Al₂O₃ in the experiments) formed by pre-anodization isa primary factor determining the shape of the resulting microcavities.In FIG. 1C, photoresist 10 is patterned onto the metal oxide 8 bycompletely encapsulating the metal/metal oxide sheet except on the topsurface at the desired positions of microcavities to be formed. Coatingthe back side (and edges) of the foil 6 with photoresist ensures that asecond anodization of the foil will not occur uniformly with respect tothe front and rear surfaces of the foil.

After the photoresist is patterned and openings are produced in theunderlying metal oxide by etching, the anodization process is continuedin FIG. 1D until the foil 6, 8 is breached beneath each of the openingsin the photoresist and microcavities 12 are formed in the foil 6, 8. Themicrocavities 12 are formed in the foil 6, 8 with a non-uniformcross-section as indicated by sidewalls 14 in FIG. 1D. FIG. 1E shows across-section of the foil that remains after the photoresist and metaloxide of FIG. 1D have been removed by etching. Much of the originalmetal is gone, having been converted into metal oxide. The microcavitysidewalls 14 are not vertical because anodization from the rear surfaceof the foil 6, 8 was blocked during the process of FIG. 1D by thephotoresist coating. Hence, a non-symmetrical anodization occurs. Thephotoresist and metal oxide of FIG. 1D are readily removed by etching inappropriate acids, respectively, leaving behind the metal layer 6 havingmicrocavities 12 with the desired shape. Although the drawing of FIG. 1E(and 1F) implies that the cavity sidewalls are linear, that need not bethe case. The precise profile of the microcavity sidewall is determinedby the thickness of the metal-oxide layer 6, 8 in FIGS. 1B and 1C, andthe anodization time in FIG. 1D. FIG. 2 illustrates qualitatively thecontinuous variation in microcavity sidewall profiles that is obtainableby the processing sequence of FIGS. 1A-1E. Extensive testing of theFIGS. 1A-1E process and inspection of the resulting cavities withoptical and electron microscopes has shown that arrays exhibit uniformemission and the V-I characteristics have a positive slope thateliminates the need for external ballasting

In addition to the breadth of cavity shapes that is achievable with thisinvention, the cavity sidewall morphology is extremely smooth.Measurements show that the RMS roughness of the microcavities of FIG. 1E(formed by process sequence 1A-1D) is well under 1 μm. If the thin metalsheet of FIG. 1E is anodized one final time, one obtains the microcavityarray shown in cross section in FIG. 1F. The microcavities have across-sectional profile determined by the process steps of FIGS. 1A-1Ebut in FIG. 1F the metal electrode(s) 6 are now buried in metal oxide 8.In fact, the electrode(s) 6 are all that remain of the original metalfoil 6 of FIG. 1A. It must be emphasized that the microcavity geometryand sidewall profile of FIG. 1E have been preserved in FIG. 1F. Thechange from FIG. 1E to 1F is that the wet chemical anodization processhas converted most of the metal into metal oxide 8 so that metal oxidenow lines the wall of the microcavity.

The electrode(s) 6 associated with the microcavities 12 of FIG. 1F canbe interconnected in patterns that are controllable. The degree ofanodization and the microcavity spacing determine the patterning ofelectrode interconnections between microcavities that occursautomatically during the course of anodization. The anodization processand microcavity placement determine whether adjacent microcavities in anarray are electrically connected or not.

As seen in FIG. 1F, the thickness of the electrode 6 is the largest inproximity to a microcavity but decreases away from the microcavity.Although not seen in the cross-section of FIG. 1F, each electrode 6surrounds each respective microcavity and is azimuthally symmetric (ifthe cavities 12 have a circular cross-section). Also, the layer ofmetal-oxide dielectric 8 exists between the inner edge of electrode 6and the wall of the microcavities 12.

The exact shape of the microcavities 12 produced in the foil 6 by theprocesses of FIGS. 1A-1F is a function of the foil thickness, initialanodization time (and, hence, oxide thickness), and second anodizationtime. In an experimental example, microcavities were formed with aslightly curved taper. In other experiments, bowl-shaped (parabolic)microcavities were formed. As one example, microcavities formed inAl/Al₂O₃ had an upper aperture with a diameter of 135±5 μm whereas thediameter of the aperture at the base of the microcavities was 76±4 μm.The uncertainty in each measurement represents one standard deviation.

Optical micrographs were recorded of 50×160 arrays of microcavitiesdevices fabricated with 2 min. of initial anodization. In fabricatingthese devices, 50×50 μm² square apertures were opened in the photoresistas shown in FIG. 1C. After anodization, however, the microcavitiesformed are circular when viewed from above. Consequently, once the foilis finally anodized, two circles associated with each microcavity couldbe seen in plan view SEM images. The larger diameter of the two was theupper aperture of the microcavity and the smaller diameter is the loweraperture or back side of the cavity. Other images taken of completedmicrocavities with buried and self-patterned electrodes showed that theelectrodes do, indeed, surround each microcavity and are disposed in aplane that is generally perpendicular to the axis of the microcavities.

FIG. 3A is schematic diagram of a lamp formed from an array 20 ofmicrocavity devices in a thin metal and metal oxide sheet. The array 20includes microcavities 12 having sidewalls with the desired profile andisolated from thin metal electrodes 6 by oxide 8. A second, commonelectrode 22 is formed in a second thin sheet that includes theelectrode 22 and an encapsulating layer of metal oxide 24. The commonelectrode 22 and metal oxide sheet is preferably formed from a thinmetal foil that has been anodized to encapsulate the metal foil 22 inthe metal oxide 24. The lamp is packaged in thin packaging layers 26, 28to seal vapor, gas or mixtures of gases and/or vapors in themicrocavities. Application of a time-varying voltage of the propermagnitude between the electrodes 6 and 22 ignites and sustains plasmawithin the microcavities.

The packaging layers can be selected from a wide range of suitablematerials, which can be completely transparent to emission wavelengthsproduced by the microplasmas or can, for example, filter the outputwavelengths of the microcavity plasma device array 10 so as to transmitradiation only in specific spectral regions. Example materials includethin glass, quartz, or plastic layers. The discharge medium can be at ornear atmospheric pressure, permitting the use of a very thin glass orplastic layer because of the small pressure differential across thepackaging layers 26 and 28, which can also be a single layer thatsurrounds the entire array. Polymeric vacuum packaging, such as thatused in the food industry to seal various food items, can also be usedas a packaging layer.

It is within each microcavity 12 that a plasma (discharge) will beproduced. The first and second electrodes 6, 22 are spaced apart adistance from each other by the respective thicknesses of their oxidelayers. The oxide thereby isolates the first and second electrodes fromone another and, additionally, isolates each electrode from thedischarge medium (plasma) contained in the microcavities 12. Thisarrangement permits the application of a time-varying (AC, RF, bipolaror pulsed DC, etc.) potential between the electrodes to excite thegaseous or vapor medium to create a microplasma in each microcavity 12.

The benefit of patterning the electrode 22 a is that the capacitance ofthe array is reduced dramatically. Furthermore, the structure of FIG. 3Ballows for addressing of individual microcavities. FIG. 3B shows anotherarray of microcavity plasma devices that includes a second electrode 22a that provides for addressing of individual microcavities 12 in thearray 20. The second electrode 22 a can be formed by photolithographyfollowed by uniform (from both sides of a metal foil) anodization, orcan be formed by anodizing a patterned foil that has holes formed byconventional methods.

FIG. 4 presents V-I characteristics of an array of microcavity plasmadevices of the invention operating in 400, 500, 600 and 700 Torr of Ne.The performance of the array at the four pressures is highly similar.Slight bending of the array was apparent, but that can be eliminated bythe use of stress reduction techniques disclosed in Eden et al., U.S.patent application Ser. No. 12/152,550, and PCT ApplicationPCT/US08/06226, both filed May 15, 2008, and entitled Arrays ofMicrocavity Plasma Devices with Reduced Mechanical Stress. In thatapplication, various stress reduction strategies are disclosed. Stressreduction can be realized by various geometries and structures,including voids between rows of microcavities and support ribs formed ofphotoresist between microcavities on one or both sides of the array ofmicrocavities. Conducting a symmetrical final anodization can alsoprovide stress reduction. With the stress reduction strategies, evenlarge arrays of microcavity plasma devices can be kept almost perfectlyflat, which provides improved emission uniformity over an array. Withproper care given to keeping the array flat, the emission fromdevice-to-device is quite uniform. FIG. 5 presents V-I characteristicsof an array of microcavity plasma devices of the invention operation inNe/Xe mixtures with Xe concentrations of 10%, 20%, 30%, 40%, 50%, and67%. The V-I characteristics of FIG. 4 and in FIG. 5 show that thesearrays are well-behaved. That is, the V-I characteristics have apositive slope that eliminates the need for external ballasting.

FIGS. 6A and 6B show another preferred embodiment array of microcavityplasma devices that is similar to the array in FIG. 3B, but includesbowl-shaped microcavities 12. The array of FIGS. 6A and 6B is labeledwith reference numbers used in FIG. 3A. In addition, the electrodes 6are illustrated as being interconnected, which can be accomplished bycontrolling the microcavity spacing and anodization, as discussed above.Thus, the four bowl-shaped (parabolic wall profile) microcavities ofFIGS. 6A and 6B are electrically interconnected, as best seen in thepartial blow-up view in FIG. 6B. Also, the electrodes 6 near themicrocavity walls have the same shape as the microcavity walls and theinterconnects will become thinner further away from the microcavities12. Arrays of the invention have many applications. Addressable devicescan be used as the basis for both large and small high definitiondisplays, with one or more microcavity plasma devices forming individualpixels or sub-pixels in the display. Microcavity plasma devices inpreferred embodiment arrays, as discussed above, can produce a plasma tophotoexcite a phosphor so as to achieve full color displays over largeareas. An application for a non-addressable or addressable array is, forexample, as the light source (backlight unit) for a liquid crystaldisplay panel. Embodiments of the invention provide a lightweight, thinand distributed source of light that is preferable to the currentpractice of using a fluorescent lamp as the backlight. Distributing thelight from a localized lamp in a uniform manner over the entire liquidcrystal display requires sophisticated optics. Non-addressable arraysprovide a lightweight source of light that can also serve as a flat lampfor general lighting purposes. Arrays of the invention also haveapplication, for example, in sensing and detection equipment, such aschromatography devices, and for phototherapeutic treatments (includingphotodynamic therapy). The latter include the treatment of psoriasis(which requires ultraviolet light at ˜308 nm), actinic keratosis andBowen's disease or basal cell carcinoma. Inexpensive arrays sealed inglass or plastic now provide the opportunity for patients to be treatedin a nonclinical setting (i.e., at home) and for disposal of the arrayfollowing the completion of treatment. These arrays are also well-suitedfor photocuring of polymers which requires ultraviolet radiation, or aslarge area, thin light panels for applications in which low-levellighting is desired.

While specific embodiments of the present invention have been shown anddescribed, it should be understood that other modifications,substitutions and alternatives are apparent to one of ordinary skill inthe art. Such modifications, substitutions and alternatives can be madewithout departing from the spirit and scope of the invention, whichshould be determined from the appended claims.

Various features of the invention are set forth in the appended claims.

The invention claimed is:
 1. An array of microcavity devices,comprising: a first electrode, the first electrode being a thin metalfoil or film including a plurality of non-uniform cross-sectionmicrocavities therein and being encapsulated in oxide of the metal ofthe thin metal foil; a second electrode being a thin metal foilencapsulated in oxide that is bonded to the first electrode, the oxidepreventing contact between the first and second electrodes; at least onepackaging layer that contains discharge medium in the microcavities. 2.An array of claim 1, wherein the microcavities have bowl shapedsidewalls.
 3. The array of claim 1, wherein the microcavities havetapered sidewalls.
 4. The array of claim 3, wherein the taperedsidewalls have a linear taper.
 5. The array of claim 1, wherein saidfirst electrode comprises a plurality of interconnected electrodes. 6.The array of claim 5, wherein said second electrode comprises aplurality of second electrodes arranged to permit addressing of saidnon-uniform cross-section microcavities.
 7. The array of claim 1,wherein the thin metal foils of the first and second electrodes comprisealuminum and the oxide of said first and second electrodes comprisesaluminum oxide.
 8. The array of claim 1, wherein the thin metal foils ofthe first and second electrodes comprise titanium and the oxide of saidfirst and second electrodes comprises titanium dioxide.
 9. The array ofclaim 1, wherein the packaging layer is one of a glass or polymer.
 10. Amethod for forming an array of microcavity devices, comprising:pre-anodizing a metal foil or thin film; patterning photoresist onto theanodized metal foil or film to encapsulate the anodized foil or filmexcept on a top surface at desired positions of microcavities;conducting a second anodization or electrochemical etching to form thenon-uniform cross-section microcavities; removing the photoresist andmetal oxide; conducting a final anodization so as to line the cavitieswith metal oxide and completely bury the metal electrodes in metaloxide.
 11. The array of claim 1, wherein the first electrode consists ofthe thin metal foil or film including a plurality of non-uniformcross-section microcavities therein and being encapsulated in oxide ofthe metal of the thin metal foil.